Create Desktop Apps. With HTML, CSS And JS: Appcelerator Titanium

Appcelerator Titanium (now known as TideSDK) is an open source platform that enables you to create rich desktop applications with web technologies like HTML, CSS, Javascript as well as Flash and Silverlight.

Appcelerator SDK is used to create applications or any third-party Ajax library / framework can be used too.

Tip: To make it clear, it is very similar to Adobe AIR but open source.

Titanium also comes with a simple command-line interface (CLI) for creating projects and packaging them into an executable.